commit c69e8d9 (CRED: Use RCU to access another task's creds and torelease a task's own creds) added non rcu_read_lock() protected accessto task creds of the target task in set_prio_one().

The comment above the function says: * - the caller must hold the RCU read lock

The calling code in sys_setpriority does read_lock(&tasklist_lock) butnot rcu_read_lock(). This works only when CONFIG_TREE_PREEMPT_RCU=n.With CONFIG_TREE_PREEMPT_RCU=y the rcu_callbacks can run in the tickinterrupt when they see no read side critical section.

There is another instance of __task_cred() in sys_setpriority() itselfwhich is equally unprotected.

Wrap the whole code section into a rcu read side critical section tofix this quick and dirty.

Will be revisited in course of the read_lock(&tasklist_lock) -> rcucrusade.